1. Field of the Invention
This invention relates to photographic apparatus of the self-developing type.
2. Description of the Prior Art
Photographic apparatus of the self-developing type are well known and generally comprise, e.g., a camera having a pair of pressure-applying members through which an exposed film unit is advanced while a processing composition is spread across a photosensitive layer of the film unit to initiate formation of a visible image in the film unit. The treated film unit is then advanced to a lighttight chamber wherein it remains for a period of time sufficient for a visible image to be substantially formed therein. Examples of cameras of the foregoing type may be found in U.S. Pat. Nos. 2,873,658; 3,396,647; and 3,537,370. Each of these patents shows a camera having a chamber for receiving at least a portion of a film unit as it is advanced from between a pair of rollers. The chambers in the first two mentioned patents function to prevent further exposure of a treated film unit to the ambient light while the chamber in U.S. Pat. No. 3,537,370 cooperates with a curtain in the form of a coil of opaque sheet material to prevent further exposure of an untreated film unit. In the latter patent, the leading edge of a film unit is adapted to engage a member on an end of the sheet and uncoil the curtain as they move together passed a viewfinder, thereby preventing further exposure of the film unit to light passing through the viewfinder. However, while the foregoing arrangements performed their function well, the chambers added to the overall dimensions of the cameras thereby placing them at an economic disadvantage relative to similar cameras of a more compact design.
Lately, there has been disclosed a family of film units of the self-developing type which can be advanced into the ambient light substantially immediately after the processing composition has been spread across at least a predetermined length of the film unit. One example of this type of film unit is described in U.S. Pat. No. 3,415,644 granted to Edwin H. Land on Dec. 10, 1968. Film units of this type enhance the compactness of cameras with which they are adapted to be used in that the cameras may be designed to advance a section of the film unit which has been treated with a processing composition from the camera while a section of that film unit yet untreated with the processing composition still remains within the camera.
In order for such an arrangement to be possible, the aforementioned film unit includes a relatively thin, substantially light transparent layer or element, which may comprise a polyester, such as a polymeric film derived from ethylene glycol terephthalic acid, or a cellulose derivative such as cellulose triacetate, defining an outside face or surface of the film unit, and a rupturable pod of processing composition that includes an opacifying constituent. During exposure operations image-carrying light rays from the camera's lens pass through the transparent surface layer before impinging upon a photosensitive layer of the film unit. As the film unit is subsequently advanced between a pair of spread rollers and towards the film exit means of the camera, the spread rollers progressively distribute a mass of the processing composition intermediate predetermined layers of the film unit. Thus, those portions of the film unit progressively exposed to ambient light have been treated by the processing composition with its opacifying constituent which precludes ambient light subsequently incident upon the transparent layer and which is actinic to the photosensitive layer from reaching adjacent or underlying sections of the photosensitive layer. Once the diffusion transfer process has been completed, the developed image is viewable through the aforementioned transparent element.
One of the considerations in camera systems of the aforementioned type is a phenomenon generally referred to as "light-piping." More specifically, light-piping comprises ambient light, which is actinic to the photosensitive layer, incident upon any portion of the transparent material exteriorly of the camera during film processing operations being conducted thereby to portions thereof adjacent yet untreated sections of the exposed photosensitive layer of the film unit. Such light rays can cause undesirable "fogging" of those yet untreated sections of the film unit's photosensitive layer.
To understand this phenomenon more clearly, it should be recognized that the aforementioned film unit comprises an opaque dimensionally stable layer or support element positioned on the side of its photosensitive layer opposed from its light transparent layer. Thus, once the processing composition including its opacifying constituent has been spread across the photosensitive layer, that photosensitive layer is protected from ambient light incident upon the film unit which is actinic to the photosensitive layer, and its development process may therefore be completed when the film unit itself is exposed to ambient light. However, in the aforementioned camera arrangement, a treated section of each such film unit is exposed to ambient light while a section thereof remains on the opposite side of the spread rollers within a light protected chamber of the camera and is yet untreated with the processing composition. While at such times ambient light incident upon the transparent layer of the treated section of the film unit cannot pass through the layer of processing composition to adjacent portions of the exposed photosensitive layer, a portion of these light rays may be reflected internally of the light transparent layer and scatter or diffuse lengthwise therealong into the light protected camera chamber until it reaches a point within the light transparent layer in advance of the composition being spread intermediate the film unit's transparent and photosensitive layers. Since there is no opacifying agent intermediate the light transparent layer and the adjacent portion of the exposed photosensitive layer at this point, these light rays, if actinic to the photosensitive layer, will have the effect of further exposing, or fogging, the yet untreated section of the photosensitive layer.
One solution to this light-piping problem as disclosed in copending U.S. Patent application Ser. No. 419,808 by Edwin H. Land entitled NOVEL PRODUCTS AND PROCESSES filed on Nov. 28, 1973 is to include an opacifying pigment in the light transparent or conducting layer itself. Such an opacifying pigment may comprise carbon black particles in such amounts as to offer little resistance to light rays passing therethough in its relatively thin (thickness) dimension, i.e., so as to not materially impair the ability to expose the photosensitive layer therethrough nor the ability to subsequently view therethrough the final image produced in the image-receiving layer or element. However, at the same time the amount of such pigment is sufficient to substantially preclude ambient light which is actinic to the photosensitive layer from being piped lengthwise through the light transparent layer to sections thereof in advance of the processing composition. In other words, when the camera lens is operative to expose the photosensitive layer, the image-carrying light rays pass through a very thin section of the light transparent element of the film unit and hence the opacifying pigment therein offers little resistance to such light rays. Similarly, the opacifying pigment does not materially affect the ability to view the final image through the transparent layer in which it is retained. However, ambient light rays attempting to pass through the same light transparent layer of the film unit must be light-piped through a much larger distance thereof, i.e., in its lengthwise direction, before reaching untreated portions of the photosensitive layer and hence the opacifying pigment is highly effective in precluding the passage of those light rays which are actinic to the photosensitive layer therethrough.
Aforementioned copending application Ser. No. 419,808 discloses densities of opacifying pigments employed in light transparent layers of particular film units which have proven suitable for resolving the light-piping problem without materially diminishing the quality of the finished photographic print. However, it will be recognized that it is highly desirable to minimize the light resistant characteristic of the film unit's light transparent layer during normal film exposure operations and therefore desirable to minimize the density of the opacifying pigment in the light transparent layer whenever this can readily be accomplished while still precluding the light-piping phenomenon and without compromising the size of the camera structure. It is to this end that one aspect of the present invention is directed.